We have a book called "It's Not the Stork" (ages 4+), which we got when dd started asking more detailed questions around age 5. I don't think you can answer the question without mentioning sex--that IS how the sperm gets to your egg--but we found the book very helpful for finding age-appropriate language. The book doesn't focus on sex, but on the differences between boys and girls and how babies grow in the womb.